1. Field of the Invention
The present invention relates generally to a surface-wave filter, and more particularly, to a surface-wave filter which can be switched to different transmission bands.
2. Description of the Related Art
Surface-wave filters are known, for example, from the data book, "Oberflaechenwellenfilter fuer Fernsehanwendungen", Datenbuch 1987/1988, published by Siemens AG. Surface-wave filters generally are integrated, passive components having band-pass filter characteristics. Surface-wave filters function based on the interference of mechanical surface waves that propagate along the surface of a piezo-electric material. The structure of surface-wave filters is characterized by a single-crystal, piezo-electric substrate, preferably a lithium niobate substrate, onto which a metal layer, normally an aluminum layer, is applied, for example, by vapor deposition. Piezo-electric input and output transducers, also referred to as interdigital transducers, are formed from this metal layer by, for example, by photo etching techniques.
The interdigital transducers are composed of a plurality of metallic, comb-like electrodes. More specifically, the electrodes each have a bus-bar as its base with fingers extending perpendicularly from the bus-bar and overlapping with like fingers of another comb-like electrode of a different polarity in an active region of each of the transducers. The substrate per se is fastened on a metal carrier. Bond wires provide the contact for the input and output transducers to terminals or pins leading to the outside.
During operation of surface-wave filters, an electrical signal which is input to the transducer is converted into a mechanical surface wave that runs along the surface of, or in a surface-proximate layer of, the substrate to an output transducer. The output transducer, in turn, converts the surface wave into an electrical signal. Standard transducers have interdigital structures with transit time effects and are distinguished by highly frequency-dependent properties so that, by appropriate formation of the finger structures of the transducers, a filter effect (i.e. the transmission band) for electrical signals is achieved.
The filter effect of the surface-wave filters is thereby essentially determined by the transmission loss, or transmission curve, which defines the transmission band. The transmission loss is the attenuation of the transmitted signal over the frequency and is at a minimum in the filter transmission band. Given a known single-crystal substrate, and assuming that the fundamental structure of the filter, such as the type and position of the transducers as well as the utilization of further components suitable for signal processing of surface waves, is fixed, then the transmission band of the filter can be varied only by modifying the internal structure of one or both transducers. For example, the finger spacing, the finger aperture or the weight of the transducer fingers can be varied to achieve variation of the transmission band. Of course, it may also be possible to vary the transmission band on the basis of external wiring of the filter.
In certain instances, for example, in multi-standard television receivers in which, for example, television transmissions that are based on some other standard, such as the I-standard, the L-standard or the M-standard, can be received in addition to PAL television transmissions, an IF filter having a transmission band corresponding to the standard is required. This also applies to NTSC transmissions. The user, or possibly even the manufacturer of the television receiver, switches to the corresponding IF filter, or selects from the possible standards, depending upon the standard being received. Included among the possible filters are what are known as double video filters having two video channels with different video bandwidths and each having two inputs and two outputs. The user alternately switches to one or the other of these two channels.